package stack_queue;

public class MyQueue {

    static class ListNode {
        public int val;
        public ListNode prev;
        public ListNode next;

        public ListNode(int val) {
            this.val = val;
        }
    }

    public ListNode first = null;
    public ListNode last = null;

    public int usedSize = 0;

    public void offer(int val) {
        ListNode node = new ListNode(val);
        if(first==null){
            first = last = node;
            usedSize++;
            return;
        }
        node.next = first;
        first.prev = node;
        first = node;
        usedSize++;
    }

    public int poll() {
        if (first == null){
            usedSize--;
            return -1;
        }
        last.prev.next = null;
        last = last.prev;
        usedSize--;
        return 1;
    }

    public int peek() {
        if(first == null){
            return -1;
        }
        return first.val;
    }

    public boolean isEmpty() {
        return usedSize == 0;
    }

}
